The present invention relates to a fuel rod for a nuclear plant and a plenum spring arranged to be provided in a fuel rod. The fuel rod comprises a cladding tube sealed by a first end plug at a first end and a second end plug at a second end, a plurality of fuel pellets stacked on each other inside the cladding tube such that they form a column of pellets and a plenum spring provided in a space of the fuel rod in order to, at least during transportation, hold the column of pellets with a spring force towards the second end of the cladding tube and wherein the plenum spring comprises a first length variable part arranged to abut the uppermost located fuel pellet in the column of pellets with an end portion and a second part arranged to allow engagement of the plenum spring against an inner surface of the cladding tube by a radially outwardly directed pressure.
Plenum springs are arranged in fuel rods for the purpose of holding the fuel pellets, which are arranged inside the cladding tube, at place during the transportation, which occurs before the fuel rods are provided in a reactor vessel. During the transportation, the fuel rods take at least partly a horizontal position and the task of the plenum spring is to prevent that the column of pellets completely or partially is displaced inside the cladding tube. If such a displacement of the column of pellets will occur, the risk exists that one or several fuel pellets is inclined inside the cladding tube and when the fuel rods is provided in a vertical position inside a reactor vessel such inclined fuel pellets prevent that the column of pellets is displaced to a correct position for operation. Consequently, the function of the plenum spring is to prevent that such a displacement or a split of the column of pellets arises during the transportation.
During the operation of a nuclear plant, fission gases are accumulated in the space above the fuel pellets in the cladding tube. Therefore, the plenum spring, which is arranged in this space, ought to take up a volume as small as possible in order not to restrict the space of the fission gases. An other advantage of using a small plenum spring is that it may be arranged at a distance from the upper end of the cladding tube where an end plug normally is welded for sealing the cladding tube. For that reason, special measures for preventing that the spring material melts and is mixed with the material of the cladding tube in the weld joint formed do not need to be taken. This also increases the space for fission gases in addition to reducing the costs.
Plenum springs have been developed which allow engagement of the plenum spring against the inside of the cladding tube at a suitable distance above the column of pellets such that a length variable springy part of the spring is allowed to act against the uppermost fuel pellet in the column of pellets for holding the column of pellets in place in the fuel rod during the transportation. JP 10020061 shows an example of such a plenum spring. During the operation of a nuclear plant, the fuel pellets are heated and the column of pellets is extended. The length variable part of the plenum spring is thereby compressed and the pressure of the plenum spring against the inside of the cladding tube increases until the friction in the contact surface between the plenum spring and the inside of the cladding tube is overcome such that the plenum spring is released. If the plenum spring does not lose its grip against the inside of the cladding tube before the pressure becomes too heavy, the cladding tube risks to be deformed.